An aerosol impactor and monitor has a plurality of impactor stages, each of which will receive an aerosol and classify the aerosol according to particle size. The impactor stages have nozzle plates and impaction plates that reduce the effects of cross flow under high flow volumes, as well as including monitors for determining pressure drop to permit analyzation of performance of the impactors. The impaction plates are mounted so that they can be rotated within the separate impactor chambers through the use of magnetic attraction drives to eliminate the need for rotating seals and yet obtain the benefit of the rotatable impaction plates. pressure sensors are used for determining pressure drop across nozzle plates, both to insure evaluation of the flow rate through the impactor and also to determine the condition of the nozzles.
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13. A particle classifier comprising:
a housing, said housing having a gas inlet and a gas outlet; a flow metering stage in the housing; a plurality of individual classification stages, said flow metering stage and said classification stages each comprising a nozzle plate having a plurality of nozzles of substantially the same size and shape, wherein the nozzles of the flow metering stage have a diameter greater than 0.3 mm. and the nozzles of at least one of the classification stages have a diameter less than 0.3 mm.; a flow controller to maintain a gas flow at a desired value, the gas flow passing through the flow metering stage and the classification stages in series flow; and a pressure transducer connected to measure the differential pressure across at least one nozzle plate for determining the operating condition of the at least one nozzle plate.
1. A particle classifier comprising a housing containing a plurality of individual classification stages, each classification stage classifying different size particles, said housing having a gas inlet and a gas outlet, a flow controller to maintain a gas flow from the gas inlet to the gas outlet at a desired value, each classification stage comprising a nozzle plate, each nozzle plate having a plurality of nozzles of substantially the same size and shape, which are different from the size and shape of the nozzles of other nozzle plates, one of the nozzle plates comprising nozzles sized to provide a flow metering stage, the gas flow passing through the flow metering stage nozzle plate and the other nozzle plates of classification stages in series flow, a first pressure transducer connected to measure the differential pressure across the one nozzle plate for determining the flow therethrough and a second differential pressure transducer for measuring the differential pressure across at least one additional nozzle plate for determining the operating condition of the at least one additional nozzle plate.
11. A particle classifier comprising a housing containing a plurality of individual classification stages, said housing having a gas inlet and a gas outlet, a flow controller to maintain a gas flow from the gas inlet to the gas outlet at a desired value, each classification stage comprising a nozzle plate having a plurality of nozzles of a different size from nozzles of other nozzle plates, the gas flow passing through the classification stages in series flow, a pressure transducer connected to measure the differential pressure across at least one nozzle plate for determining the operating condition of the at least one nozzle plate, wherein the flow controller includes a computer, a temperature sensor connected to measure ambient temperature, and a pressure sensor to measure barometric pressure, the computer calculating a flow rate from the flow controller based on the measured temperature, barometric pressure, and the differential pressure sensed by connecting the pressure transducer across said at least one nozzle plate, said at least one nozzle plate containing nozzles larger than 0.3 mm in diameter, and providing an electrical signal to set the flow rate to a specific set point value.
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This invention relates to a cascade aerosol impactor for classifying aerosol particles that includes pressure sensors for monitoring the functions of the impactor, and further includes mounting structure that permits rotating impaction plates without mechanical drives that require seals. The nozzles used are also constructed to reduce cross flow effects.
Inertial impactors are widely used for measuring the size distribution of aerosols. For purpose of this invention, particles suspended in a gas are referred to as an aerosol. The gas is usually air, but other gases such as nitrogen, oxygen, argon, helium, etc. can also be the suspending gaseous medium. The particles can be a solid, a liquid, or a mixture of both. The particle size is usually between 0.002 μm and 100 μm.
Inertial impactor, aerosol impactor, or impactor all refer to an aerosol sampling or collection device that separates aerosol particles from the gaseous medium in which they are suspended by the inertial effect of the particles. The device usually uses a nozzle to accelerate the gas to a high velocity and direct the gas jet against an impaction plate to cause particle impaction on the plate. Particles will impact only when their size is larger than a certain critical value, while smaller particles with insufficient momentum or inertia will be carried by the gas flow around the plate to an exit and escape collection.
The critical particle size at which particle collection occurs is referred to as the impactor cut-point. The cut-point particle diameter of an impactor can be varied by varying the nozzle size and gas velocity. Smaller nozzles and higher gas velocities will produce smaller cut-point particle diameters. The impactor cut-point is also affected by the gas viscosity, the shape of the nozzle, as well as the nozzle-to-plate distance. In an ideal impactor all particles larger than the cut-point are collected with 100% efficiency while smaller particles are not collected. In a real impactor, particle impaction does not occur ideally at a single particle size. The transition from zero to 100% particle collection usually occurs over a range of particle sizes. The narrower this range, the sharper the impactor cut-size characteristics. The ideal impactor is then an impactor with a perfect cut value. In a real impactor with less than a perfect sharpness-of-cut, the cut-point is usually defined as the particle diameter at which 50% of the particles are collected.
The prior art impactor just described is a single stage impactor. It consists of a single nozzle plate carrying one or more nozzles in parallel and an adjacent impaction plate. Several single-stage impactors can be arranged in series to form a cascade impactor. Cascade impactors are designed so that large particles are collected first, followed by smaller and smaller particles between an inlet and an outlet. There is usually a final filter to collect small particles below the cut-point of the last impactor stage. For instance, in a three-stage impactor with cut-point diameters of, say 10, 3 and 1 μm, particles larger than 10 μm are removed by the first, 10-μm cut stage. Subsequently, particles in the 3-to-10 μm and the 1-to-3 μm ranges are removed by the 3-μm and 1-μm cut stages, respectively. The final filter then collects particles smaller than 1 μm.
The cascade impactor is very useful for size distribution analysis of aerosol particles. Particulate air pollutants, aerosols in the work place environment, as well as other aerosols of practical interest are usually polydisperse, with particle sizes spread over a wide range of values. Cascade impactors can be used to separate particles by size into narrower intervals for analysis. The size-fractionated particles can then be analyzed gravimetrically to determine their mass size distribution. Alternatively, the particles can be analyzed chemically to determine the chemical composition of the particles as a function of particle size. Cascade impactors are widely used in air pollution studies to determine the physical and chemical properties of the airborne particles, assess their potentially harmful health effect, or determine the origin of the particles for pollution abatement or control purposes.
In the schematic prior art diagram of
In recent years, demands for increased accuracy and precision for aerosol measurement have led to the development of cascade impactors with a high volumetric flow rate and large number of impactor stages. For instance, the Micro-Orifice Uniform Deposit Impactor, sold under the trademark MOUDI™, manufactured by the MSP Corporation of Minneapolis, Minn., the assignee of this application, comprises eight (8) or ten (10) impactor stages with nominal cut-point particle diameters that range between 18, 10, 5.6, 3.2, 1.8, 1.0, 0.56, 0.32, 0.18, 0.1 and 0.056 μm. In the final stages, nozzle diameters as small as 50 μm are used. To provide the needed 30 liter-per-minute sampling flow rate, as many as 2,000 nozzles are used in some stages.
The need for increased measurement sensitivity in air pollution research and for other applications has created the need for impactors with flow rates larger than the 30 liters-per-minute. To design impactors with higher flow rates, even larger number of nozzles need to be used. To create a cascade impactor with, say, 90 liters-per-minute sampling flow rate, and similar operational pressure drop characteristics, the number of small nozzles needs to be increased by a factor of 3. Thus, 6000 nozzles need to be used in the final stages of such high flow MOUDI™ cascade impactors.
In designing impactors with large numbers of very small nozzles, it is important to consider the effect of cross flow in the impactor. As will be explained, when a nozzle plate with a large number of nozzles is provided, the gas flow through the outer nozzles, must pass radially outward across the surface of the nozzle plate between it and the impaction plate. This outward radial gas flow is referred to as the cross-flow. The cross flow can cause the gas jets through the nozzles in the cross flow path to be deflected side ways and change their cut-point characteristics. The sharpness-of-cut of the impactor as a whole will then decrease. The cross flow effect is the greatest for nozzles located near the outer edges of the nozzle cluster. To obtain good sharpness-of-cut characteristics, it is important to consider the cross flow effect in designing high flow impactors.
The use of large number of very small nozzles also creates the practical issue of nozzle plugging during use. As aerosols are sampled by the impactor, some accumulation of particulate matter around the edge of the nozzle is unavoidable. Over time, enough material can accumulate to partially block the flow and cause the cut-point of the impactor to change. This effect, if not monitored, can lead to measurement errors. The nozzles can usually be cleaned, but cleaning, if done improperly, for instance, by using a high intensity ultrasonic cleaner, can cause damage to the nozzle plate and alter the size of the nozzles.
Another issue relating to high accuracy, high precision modern impactor is that the use of a large number of impactor stages can cause errors in the assembly of the impactors. In the MOUDI™ system referred to above, eight and ten impactor stages are used. Usually, the impactor stages must be assembled with impactor stage cut-points in a decreasing order. If, due to an operator mistake, the impactor stages are assembled incorrectly with the order of some stages reversed, erroneous data will be collected. The present invention solves the problems outlined, including the reduction of cross flow, monitoring changes in cut point and automatically detecting errors in the assembly and use of cascade impactors.
The present invention relates to a cascade impactor that provides a very accurate particle size cutoff at each of the stages of the impactor. The particle sizes are obtained by precise controlling of cross flow, and also by monitoring the performance of the nozzle plate. Additionally, enhanced operation is achieved by rotating the impaction plates utilizing a drive, which avoids the need for rotating seals between individual stages of the impactor. By monitoring the pressure drop across the individual stages and comparing it with standard or reference values of pressure drop obtained for similar nozzle plates, or obtained by calibrating the nozzle plates, any changes that indicate leaks, damage to the nozzle plates, or other detrimental factors can be determined quickly. Further, monitoring pressure also insures a check that the assembly of the various stages of the impactor is appropriate, because errors in assembly will cause changes in the pressure drop across the nozzle plates at different stages. Preset limits of pressure drop change can be set so that when the different pressure changes reach the set amount, alarms or other indicators can be activated for alerting an operator to a problem.
In one form of the invention, an optical sensor is used for providing signals indicating amount s of an aerosol provided by a drug delivery device on order to provide automatic measurement of such aerosol.
The overall construction thus insures simple operation, simple assembly and reliable, accurate classification of particles at quite "sharp" cutoff points.
For purpose of this invention, the various terms are defined as follows:
Aerosol: Aerosol means small particles suspended in a gas. The gas is usually air, but can also be nitrogen, oxygen, argon, and other types of gases. The particles can be solid, liquid or a mixture of both. The particle size is usually between 0.002 μm to 100 μm.
Impactor: Impactor, inertial impactor, and aerosol impactor all refer to devices that collect aerosol particles by virtue of the inertial effect of particles. Impactors use a nozzle to accelerate the gas flow to a high velocity. The gas jet is then directed against an impaction plate to collect the suspended particle by inertial impaction. More than one such nozzle can be formed in a nozzle plate to increase the volumetric gas flow through the device.
Nozzle and orifice: A nozzle can be a flow passage with a converging cross section to accelerate the gas flow to a high velocity. It can also be a converging-diverging nozzle, such as a Venturi. A sharp-edge orifice and other forms of flow restrictions can also be used. The cross-sectional shape of the nozzle can be circular, rectangular, or a long slit of a large length-to-width ratio. Nozzles and orifices are used interchangeably in the present invention and may have any cross-sectional shape, perpendicular to the axis or plane of flow including, but not be limited to, that of a circle.
Nozzle throat: When the nozzle has a varying cross-section, the smallest cross-section is referred to as the throat. In the case of a nozzle with a circular cross-section, the diameter of the throat is the diameter of the smallest circular cross-section in the nozzle.
Aerodynamic equivalent diameter: An aerodynamic equivalent diameter of an aerosol particle of an arbitrary size, shape, and density is the diameter of a unit density sphere having the same settling velocity as the particle in question. The cut-point diameter of an impactor referred to in this description is based on the aerodynamic equivalent diameter of the particles.
Cascade impactor: A cascade impactor is a series of impactors formed in a single device. Each impactor in the series is then referred to as an impactor stage. The individual impactor stages are usually arranged in a descending order in stage cut-point. The particles collected by the individual stages can be weighed to determine the mass size distribution, or analyzed chemically to determine their chemical composition as a function particle size.
Computer: Computer is an electronic device for computing and data processing. It includes the popular, general-purpose Personal Computers (PC), and special-purpose electronic devices, such as the microprocessor, the microcontroller, and the like, that are used for data acquisition and processing, and special control functions.
Optical Aerosol Sensor: Optical aerosol sensor is an aerosol sensing device based on the scattering or transmission of light in an aerosol cloud. It measures the concentration of particles in the cloud by the light it scatters or transmits. It includes a light source projecting a beam of light through the cloud, a window or lens system to allow the scattered or transmitted light to fall onto a light sensor. The light source is usually a laser, an incandescent lamp, or a discharge lamp. The light sensor can be a solid state light detector commonly referred to as a photodiode, a photomultiplier tube, or a charge-coupled-device (CCD) of the type commonly used in still or video cameras. The wavelength of light is not limited to the 0.4 μm of 0.7 μm range that can be seen by the human eye, but include electromagnetic radiation in the range of the infrared, i.e. >0.7 μm in wavelength, or the ultraviolet, i.e. >0.4 μm in wavelength.
The schematic diagram of a single prior art impactor stage 20 shown in
As illustrated in
One way to reduce the cross-flow effect is to create flow channels or grooves 30 on a nozzle plate 32 and flow channels or grooves 36 on an associated impaction plate 38, as shown in
The same concept of flow channeling can also be applied to the case of nozzle clusters that are not circular in shape.
Although it is generally preferred to use identical nozzle clusters to produce a high flow rate impactor, the basic concept of flow channeling for controlling cross flow is not restricted to identical nozzle clusters. Various cluster sizes and shapes can be used in an impactor provided the flow from each cluster can be properly channelled away.
In the Micro-Orifice, Uniform Deposition Impactor (MOUDI™) now produced by the MSP Corporation, the uniform deposit feature of the impactor is obtained by using nozzle plates having a substantially uniform distribution of nozzles and rotating the nozzle plates relative to the impaction plates while flow is carried through the nozzles. This rotation causes the particle deposits from the individual nozzles to be spread out uniformly over circular bands on the impaction plates. Using a uniform distribution of nozzles in the nozzle plates, the particle deposit on the impaction plate also becomes substantially uniform. Uniform particle deposit reduces the probability of particle bounce and re-entrainment. It is also demanded by certain chemical analysis techniques, such as X-ray fluorescence.
The rotary motion of the MOUDI™ impactor is achieved by a set of mechanical gears and restraining hooks to produce the relative rotation between the nozzle plate and impaction plate. Such mechanical drive for obtaining rotation has worked satisfactorily, but is not the most ideal or the most flexible. Possible gas leakage in the rotating seals is an issue, and the seals also need to be replaced periodically when they become worn.
The bearings 66A-66C are mounted in bearing housings 68A-68C which in turn are supported on a fixed impaction plate supports 70A-70C, respectively. The impaction plate support includes the horizontal plates with a large number of openings 72A-72C respectively, that do not restrict airflow through the individual cascade impactor chambers that are shown at 55A-55C.
The openings 72A-72C are arranged so that flow through them is substantially unobstructed, and in relation to the nozzles or nozzle openings 53A-53C in each of the nozzle plates 54A-54C, the openings 72A-72C are extremely large.
It is noted that the bearing housing supports 70A and 70B, which also then, in turn, support the impaction plates or collection substrates are mounted at the upper classification or impactor chambers 55A and 55B on the nozzle plates 54B and 54C, respectively, and the lower support 70C is supported on an end plate 74 of the housing 56. The end plate 74 has an airflow outlet tube 76 mounted thereon, as well. The airflow outlet tube is connected to a pump 75. The arms 62A-62C are relatively narrow, so they do not substantially obstruct airflow past them, and as stated are rotatably mounted in precision bearings 66A-66C so there is very little friction resisting rotation. The outer ends of each of the arms 62A-62C have dual magnets attached thereto, and as shown the arm 62A has a depending leg 80A on which a permanent magnet of high strength shown at 82A is mounted. The magnet 82A is spaced from, but is close to the impaction plate 54B that divides the chamber 55A from the chamber 55B.
The arm 62B in chamber 55B has a two section leg 80B attached thereto, with one section of leg 80B extending upwardly from arm 62B and mounting a magnet 82B, and a second section of the leg 80B depending downwardly and mounting a magnet 82C. The magnets 82B and 82C are close to the impaction plates 54B and 54C, respectively as shown, but are spaced from such plates so that they do not interfere with the rotation of the arm 62B, which again, is mounted on very low friction, high precision bearings.
The arm 62C in the chamber 55C of the cascade impactor has a two section leg 80C attached thereto at an outer end, and the leg 80C is similar to the leg 80B in that it extends in upward and downward directions from the arm on which it is mounted. The arm 80C mounts a magnet 82D at the upper end, and another magnet 82E at a lower end. The magnet 82D is close to but spaced from the underside of the nozzle plate 54C and the magnet 82E is spaced above, but close to the end plate 74 of the housing 56.
Since the arms 62A-62C are freely rotatable, the magnets 82A-82E can be used for rotationally driving the arms. In order to provide a magnet drive, a drive motor 86 is mounted, as shown schematically, on the lower side of the end plate 74, and it drives a rotating hub or pulley 88, that is rotatably mounted on the outlet tube 76 using suitable bearings 90. The hub 88 can be a cylinder, driven by a belt 92 from a pulley 94 on the output shaft of the motor 86.
The hub 88 can have a high friction surface so that belt 92 will drive against a cylindrical surface because there is very little load on the hub. The hub is used to mount a radial arm 96, which is fixed to hub 90 and which extends radially from the central axis of the tube 76. The arm 96 is fixed to the hub 90 in a suitable manner. The arm 96 has a drive magnet 98 at its outer end, which is in vertical (radial) alignment with the radial positions of the magnets 82A-83E, and in particular is aligned with the adjacent magnet 82E.
The motor 86 is mounted on a suitable support, to provide clearance for the arm 96 as it rotates, and when the motor 86 is driven under control of a controller 100 the arm 96 will rotate causing the magnet 98 to rotate and because of the orientation of the magnets 98 and magnet 82E, the magnet 82E will be caused to follow the magnet 98 and thus rotate the arm 62C and impaction plate 58C. In turn, the magnet 82D carried on the arm 62C will drive magnet 82C and arm 62B in the chamber 55, thereby rotating the impaction plate 58B. The magnet 82B will drive the magnet 82A in chamber 55A, thereby rotating impaction plate 58A. The magnetic drive is used for rotating the impaction plates underneath the individual nozzles of the respective nozzle plates. The controller 100 also can control the pump 75.
Rotary motion of arm 96 and magnet 98 is transmitted by magnetic coupling between the neighboring magnets and causes all the impaction plates 58A-58C to rotate. The housing 56, which remains stationary, and the nozzle plates, which are also stationary, arms 62A-62C and other components must be made of a non-magnetic material, such as aluminum or plastic, while the magnets 82A-82E are strong permanent magnets. The magnet drive does not require any rotating seals between impaction chambers 55A-55C. Also, different numbers of impactor stages or chambers can be provided with a single motor drive, and there is no need to design or use a different housing for different impactor stages of a cascade impactor. This results in flexibility of use as well as reduced cost of manufacturing.
The cascade impactor 104 is the same as that shown in
This reduces the number of pressure sensors needed, hence, the cost of the overall system. Ambient temperature can be sensed with a temperature sensor 126. Barometer pressure is sensed with a sensor 128; relative humidity can be sensed with a sensor 130. Flow rate can be sensed by calculating the flow based on measured pressure drop across an orifice plate. A separate flow sensor can be used. The sensors are connected to a computer 134 for signal processing and recording. The computer 134 can also generate an electrical signal based on the sensor inputs to adjust the flow by controlling the speed of pump 114 (or 75) with a speed controller 140 to one of several preset values.
The pressure drop across each impactor stage can be sensed by the individual pressure sensors and compared with standard or reference values which may be obtained by calibration at the factory or at user's standard calibration laboratory at periodic intervals.
During use in the field, when particles begin to accumulate on the small nozzles in the nozzle plates, the pressure drop across the nozzle plate increases. This increase can be detected. In addition, any leak in the system, damage to the nozzle plates due to cleaning or other causes, as well as a mistake in the assembly and operation of the impactor can also be detected automatically.
Preset pressure limits can also be established so that when the pressure difference between the measured value during use and the calibrated set point exceeds the limit, the operator will be alerted to the situation for corrective actions. The data can also be stored in the computer memory from a flow set control 142 and a pressure limit controller 144. Each pressure sensor can be individually monitored by the computer. This way, changes that have occurred during sampling can be detected and the time at which these changes have taken place will also be known. This will enable the experimenter to determine if the data are sufficiently accurate for use or need to be discarded.
Change in nozzle opening dimensions due to particle accumulation and blockage is generally not an issue when the nozzle is a few millimeter or more in diameter. For smaller nozzles, especially those found in modern precision impactors such as the MOUDI™ impactor mentioned earlier, it is important. Due to the very small nozzle diameter, the nozzle plate carrying these small nozzles must also be very thin, typically a few thousandth of an inch in thickness. Such thin nozzle plates can be easily damaged during ultrasonic cleaning. Presently, there is no convenient way of detecting the small change in nozzle diameter due to particle accumulation and/or damage during cleaning. Manual inspection by microscope is slow and labor intensive. Due to the high microscope magnification needed to see the small nozzles, the field of view is quite small, meaning that only a few nozzles can be seen and examined in a given field of view. Since micro-orifice nozzle plates with as many as 2,000 nozzles are routinely made, and as many as 10,000 nozzles may be needed in the future, the convenient and low cost method of detecting change in nozzle dimensions is described above accomplishes the objective automatically.
One of the most important causes of inaccurate or invalid data is operator error. Since cascade impactor stages each have a separate nozzle plate, they can be assembled incorrectly and in the wrong sequence. Leakage in O-ring and other types of seals used in a cascade impactor can also cause invalid data to be generated. The method described above can detect both of these problems automatically and at low cost.
One further advantage of sensing the pressure drop across the impactor stages is that the flow through a cascade impactor is often maintained by the use of a critical flow control orifice. The critical orifice shown at 150 in
A critical flow orifice 150 (
Studies show that to be suitable for use as a flow-metering stage, the nozzle diameter in a nozzle plate must be larger than about 0.3 mm in diameter. Nozzles smaller than 0.3 mm in diameter have the possibility of being partially clogged during operation due to particle accumulation around the edges of the nozzles near the entrance, thereby changing their effective opening size and hence the pressure drop. For the same reason, only nozzles smaller than 0.3 mm diameter need to be monitored to detect area changes due to particle accumulation, since large nozzles are unlikely to be clogged by particles deposition at the nozzle entrance.
Pressure drop monitoring provides a very sensitive way of monitoring changes in nozzle diameters. According to theory, the cut-point diameter of an impactor stage is determined by the dimensionless Stokes number defined as:
where u is the gas flow velocity through the nozzle, τ is the particle relaxation time, and D is the nozzle diameter. For a specific nozzle design, the value of the St is fixed. The above equation can then be used to determine how changes in nozzle flow velocity, u, and nozzle diameter, D, can affect the particle relaxation time. The particle relaxation time is a function of particle size and the gas properties. For a specific gas, such as air, the gas properties are fixed, and the relaxation time then depends only on particle size. Hence, the equation can be used to determine the effect or changes in u and D on the size of particle the impactor can collect by impaction.
To estimate the effect of dimension changes, it should be noted that a 1% reduction in nozzle diameter, D, would cause a 2% reduction in nozzle area. With the same volumetric gas flow through the nozzle, the velocity will then increase by 2%. The pressure drop will then increase by 4% based on the Bernoulli' law. Since the particle relaxation time depends on the second power of particle diameter, the cut-point diameter of the nozzle will then decrease by 1.5%. Thus, by monitoring the pressure drop across nozzle plate, a 4% change in pressure drop will correspond to an impactor cut-point change of only 1.5%. Since pressure drop changes of 1% or less can be relatively easily measured with a precision pressure transducer, cut-point diameter changes of 0.4% or smaller can be easily detected. For a 50 μm diameter nozzle, 0.4% change in diameter corresponds to a 0.2 μm change in actual dimension. Such a small change in diameter cannot be easily measured with available optical inspection tools.
Pressure drop monitoring, therefore, is most effective by choosing a nozzle plate having larger than 0.3 mm diameter nozzles as a flow metering stage, and then monitoring the pressure drop of nozzle plates with nozzles smaller than 0.3 mm for effects of particle accumulation. This way, inaccurate flow measurement as well as nozzle clogging can be simultaneously detected with confidence by pressure drop monitoring across nozzle plates.
One important application of cascade impactors is to measure the size distribution of aerosols produced for medicinal uses. In such applications, the specific chemical compound, i.e. drug, is aerosolized, which is then inhaled by the patient. The most widely used devices for producing medicinal aerosols for inhalation therapy are the metered dose inhaler (MDI) and the dry-powder inhaler (DPI). These devices produce a specific quantity of drug in aerosol form with each application, usually by depressing the device with a thumb or squeezing the device between fingers to release a puff of aerosol containing the required dose which the patient then inhales.
It is important to know both the dose, and the size distribution of aerosols produced by these devices. The aerosol size distribution is important because it determines how much and where the aerosol will deposit in the patient's respiratory system. Particles in the 3-to-10 μm range will usually deposit in the upper respiratory airways, i.e. the trachea and the bronchial airways, while smaller particles will penetrate more deeply into the lung and be deposited in the alveolar region. For instance, bronchial dilating drugs for treating asthma generally require a significant portion, if not the major portion, of the drug aerosol to be in the 3 to 10 μm range for the drug to be effectively delivered to the target site.
The operating characteristics of a MDI or a DPI is usually determined by sampling the medicinal aerosol into a cascade impactor and analyzing the quantity of aerosol collected in each impactor stage to determine both the size distribution and the total aerosol output. The impactor performance monitoring system shown in
The stable operation of a specific type of MDI or DPI is very important to insure consistent output in total dose and size distribution. An aerosol-sensing device that can sense one or more of the aerosol characteristics automatically will be a valuable tool in the design and development of MDI and DPI and quality assurance in production.
The rapid response of the optical aerosol sensor 160 makes it possible to determine the time profile of drug delivery by the MDI or DPI. The optical sensor can also be used without the USP Inlet for other applications for simultaneously sensing the aerosol concentration by optical means with optical sensor 160 while collecting the aerosol with a cascade impactor 163 at the outlet of the optical sensor or filter sampler for size distribution or total mass analyses. The collected particle sample can also be analyzed chemically to determine the medicinal content of the collected particles.
For purpose of illustration, the optical aerosol sensor is shown in more detail in
For MDI and DPI testing, aerosol particles deposited on the inside walls of the USP Inlet must be recovered and analyzed as part of the total aerosol output of the MDI or DPI. This is usually accomplished by rinsing the interior surface of the USP Inlet 162 with an appropriate solvent, such as distilled water. The solvent can then be analyzed chemically to determine its medicinal content. To facilitate this process, a transparent window and lens 190 can be formed into the walls of the inlet, making them integral parts of the Inlet. This way, the deposited material on the window can be recovered in the usual manner by removing the window and lens. At the same time the windows and lens can be cleaned each time after use.
An important application of the optical aerosol sensor and particle collector is to measure the dose uniformity of drug delivery devices such as the MDI and the DPI. For this application, the particle collector would be a filter (FIG. 14), and the device can be used with or without the USP Inlet. In the procedure currently used for testing such devices, as described in the U.S. Pharmacopoeia, the dose is measured by directing the aerosol cloud onto a filter, which is then analyzed. In a 100-dose device, i.e., a device capable of producing 100 doses, 10 separate dose samples are collected and individually analyzed. The values are then averaged to give the so-called "label-claim dosage." In this procedure, the remaining 90 doses are wasted. The procedure is laborious and time consuming. It also measures the average dose based on statistical sampling, rather than the entire content of the device.
In the present invention, the light scattering aerosol sensor measures particle concentration in the aerosol cloud as it passes through the laser beam on its way to the filter. The particle scattered light signal can be integrated with respect to time and compared with the actual mass of particles collected by the filter to provide a calibration for aerosol mass. Once calibrated, the device can be used to measure the mass output of a drug delivery device on each dose produced by the device.
To perform a dose-uniformity test, the MDI or DPI is attached to the inlet of the aerosol sensor. Each time a dose is released, the scattered light signal is obtained and stored in the computer, which then analyzes the signal to obtain an integrated scattered light value to obtain the dose. The actual dose is also collected by the filter. After the entire content of the device has been emptied following the release of 100 doses on a 100-dose device, the filter is then analyzed. The total quantity of material collected by the filter divided by the number of doses produced is then the true average dose. The dose uniformity is obtained by analyzing the stored data in the computer. This procedure requires only the analysis of one filter sample, as opposed to the 10 samples currently required by U.S. Pharmacopoeia. It is also more accurate, because the entire content of the device has been collected and analyzed.
The optical sensor shown in
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
Marple, Virgil A., Liu, Benjamin Y. H.
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